After adding a second sound-card, they both show up in Windows 7. The OS just allows one Default Recording Device, which is understandable and not a problem because in Audacity you can choose from any of the available input devices regardless of the default input device.

The one problem I had was that Audacity only allows to record from one device at the time. Opening another instance of the software just controls the recording process of the first instance.

To work around this, I opened the second instance as a different user. Use the shift-context menu to 'run as different user' or a batch file with PSExec: